The present invention relates to an apparatus for controlling a throttle valve of an engine and, more particularly, to an apparatus for controlling a throttle valve of which the valve opening degree is controlled by a motor.
The valve opening degree of the throttle valve of the engine is generally controlled by mechanical connecting means, such as a cable or a linkage connected to an accelerator pedal. In recent years, however, there has been proposed a controlling device in which the step-on amount of the accelerator pedal is electrically converted and the valve opening degree is controlled by driving a motor for driving the throttle valve in correspondence to the converted amount. In this controlling device, a valve return means is needed in order to permit the throttle valve to return to its regular set position when the motor is not driven. As the valve return means, a torsion coil spring is used into which an extended portion of the shaft of the throttle valve is inserted and which is fixed to the shaft at its one end and to the housing at its other end. In this torsion coil spring, a large hysteresis exists in the spring characteristic thereof due to contact friction between the inside surface of the spring and the shaft and due to contact friction between adjacent lines of the spring, and furthermore, once the valve begins to move, the friction developed in the spring is changed from static friction to kinetic friction so as to be reduced. In consequence, when it is intended, for example, to control the valve to open it at a valve opening degree of which the spring return force is greater than the present valve opening degree, the valve cannot begin to move unless the output power of the motor becomes larger than the resultant force of the spring return force and the frictional force. However, there is a disadvantage that once the valve begins to move, the output power of the motor becomes excessively large because the friction is changed from static friction to kinetic friction as described above.
On the other hand, in the control of the valve opening degree of the throttle valve of the kind described above, it is usual to apply at least PI control, out of PID control, so as to minimize the error of the command valve opening degree.
In order to quickly reach the command valve opening degree, in other words, in order to obtain a quick control response ability, it is enough to make large the gain of the I control. However, since the frictional force becomes smaller after the valve begins to move as described above, the gain of the I control becomes larger than is required. In consequence, the valve greatly overshoots the command valve opening degree, thereby making it impossible to obtain a high controlling accuracy. In order to obtain the derived controlling accuracy, it is sufficient to make small the gain of the I control. In this case, however, it is impossible to obtain the quick control response ability. Accordingly, the existing throttle valve makes a compromise in respect of both the controlling accuracy and the control response ability since it is difficult to obtain an excellent controlling accuracy and an excellent control response ability concurrently.